dmaengine: ioat: remove unused ‘res’

In __cleanup(), variable ‘res’ is initialized but never used, which
leads to warning with W=1

drivers/dma/ioat/dma.c: In function ‘__cleanup’:
drivers/dma/ioat/dma.c:614:28: warning: variable ‘res’ set but not used [-Wunused-but-set-variable]
    struct dmaengine_result res;

So remove it.

Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>